<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
<title>Nonterminal</title>
<link rel="stylesheet" href="../../../../../../../../doc/src/boostbook.css" type="text/css">
<meta name="generator" content="DocBook XSL Stylesheets V1.79.1">
<link rel="home" href="../../../../index.html" title="Spirit 2.5.8">
<link rel="up" href="../generator_concepts.html" title="Generator Concepts">
<link rel="prev" href="narygenerator.html" title="NaryGenerator">
<link rel="next" href="../basics.html" title="Generator Basics">
</head>
<body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF">
<table cellpadding="2" width="100%"><tr>
<td valign="top"><img alt="Boost C++ Libraries" width="277" height="86" src="../../../../../../../../boost.png"></td>
<td align="center"><a href="../../../../../../../../index.html">Home</a></td>
<td align="center"><a href="../../../../../../../../libs/libraries.htm">Libraries</a></td>
<td align="center"><a href="http://www.boost.org/users/people.html">People</a></td>
<td align="center"><a href="http://www.boost.org/users/faq.html">FAQ</a></td>
<td align="center"><a href="../../../../../../../../more/index.htm">More</a></td>
</tr></table>
<hr>
<div class="spirit-nav">
<a accesskey="p" href="narygenerator.html"><img src="../../../../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../generator_concepts.html"><img src="../../../../../../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../../../../index.html"><img src="../../../../../../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="../basics.html"><img src="../../../../../../../../doc/src/images/next.png" alt="Next"></a>
</div>
<div class="section">
<div class="titlepage"><div><div><h5 class="title">
<a name="spirit.karma.reference.generator_concepts.nonterminal"></a><a class="link" href="nonterminal.html" title="Nonterminal">Nonterminal</a>
</h5></div></div></div>
<h6>
<a name="spirit.karma.reference.generator_concepts.nonterminal.h0"></a>
            <span class="phrase"><a name="spirit.karma.reference.generator_concepts.nonterminal.description"></a></span><a class="link" href="nonterminal.html#spirit.karma.reference.generator_concepts.nonterminal.description">Description</a>
          </h6>
<p>
            A Nonterminal is a symbol in a <a class="link" href="../../../abstracts/parsing_expression_grammar.html" title="Parsing Expression Grammar">Parsing
            Expression Grammar</a> production that represents a grammar fragment.
            Nonterminals may self reference to specify recursion. This is one of
            the most important concepts and the reason behind the word "recursive"
            in recursive descent generation.
          </p>
<h6>
<a name="spirit.karma.reference.generator_concepts.nonterminal.h1"></a>
            <span class="phrase"><a name="spirit.karma.reference.generator_concepts.nonterminal.refinement_of"></a></span><a class="link" href="nonterminal.html#spirit.karma.reference.generator_concepts.nonterminal.refinement_of">Refinement
            of</a>
          </h6>
<div class="blockquote"><blockquote class="blockquote"><p>
              <a class="link" href="generator.html" title="Generator"><code class="computeroutput"><span class="identifier">Generator</span></code></a>
            </p></blockquote></div>
<h6>
<a name="spirit.karma.reference.generator_concepts.nonterminal.h2"></a>
            <span class="phrase"><a name="spirit.karma.reference.generator_concepts.nonterminal.signature"></a></span><a class="link" href="nonterminal.html#spirit.karma.reference.generator_concepts.nonterminal.signature">Signature</a>
          </h6>
<p>
            Nonterminals can have both consumed and inherited attributes. The Nonterminal's
            <span class="emphasis"><em>Signature</em></span> specifies both the consumed and inherited
            attributes. The specification uses the function declarator syntax:
          </p>
<pre class="programlisting"><span class="identifier">RT</span><span class="special">(</span><span class="identifier">A0</span><span class="special">,</span> <span class="identifier">A1</span><span class="special">,</span> <span class="identifier">A2</span><span class="special">,</span> <span class="special">...,</span> <span class="identifier">AN</span><span class="special">)</span>
</pre>
<p>
            where <code class="computeroutput"><span class="identifier">RT</span></code> is the Nonterminal's
            consumed attribute and <code class="computeroutput"><span class="identifier">A0</span></code>
            ... <code class="computeroutput"><span class="identifier">AN</span></code> are the Nonterminal's
            inherited attributes.
          </p>
<p>
            The default value is <code class="computeroutput"><span class="keyword">void</span><span class="special">()</span></code> (no consumed and inherited attributes).
          </p>
<h6>
<a name="spirit.karma.reference.generator_concepts.nonterminal.h3"></a>
            <span class="phrase"><a name="spirit.karma.reference.generator_concepts.nonterminal.attributes"></a></span><a class="link" href="nonterminal.html#spirit.karma.reference.generator_concepts.nonterminal.attributes">Attributes</a>
          </h6>
<p>
            The Nonterminal models a C++ function. The Nonterminal's consumed attribute
            is analogous to the function return value as it is the type -exposed-
            by the Nonterminal. Its inherited attributes are analogous to function
            arguments. The inherited attributes (arguments) can be passed in just
            like any <a class="link" href="../basics.html#spirit.karma.reference.basics.lazy_argument">Lazy
            Argument</a>, e.g.:
          </p>
<pre class="programlisting"><span class="identifier">r</span><span class="special">(</span><span class="identifier">expr</span><span class="special">)</span> <span class="comment">// Evaluate expr at parse time and pass the result to the Nonterminal r</span>
</pre>
<h6>
<a name="spirit.karma.reference.generator_concepts.nonterminal.h4"></a>
            <span class="phrase"><a name="spirit.karma.reference.generator_concepts.nonterminal._code__phrase_role__identifier___val__phrase___code_"></a></span><a class="link" href="nonterminal.html#spirit.karma.reference.generator_concepts.nonterminal._code__phrase_role__identifier___val__phrase___code_"><code class="computeroutput"><span class="identifier">_val</span></code></a>
          </h6>
<p>
            The <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">spirit</span><span class="special">::</span><span class="identifier">karma</span><span class="special">::</span><span class="identifier">_val</span></code> placeholder can be used in <a href="../../../../../../../../libs/phoenix/doc/html/index.html" target="_top">Boost.Phoenix</a> semantic
            actions anywhere in the Nonterminal's definition. This <a href="../../../../../../../../libs/phoenix/doc/html/index.html" target="_top">Boost.Phoenix</a>
            placeholder refers to the Nonterminal's (consumed) attribute. The <code class="computeroutput"><span class="identifier">_val</span></code> placeholder acts like an immutable
            reference to the Nonterminal's attribute.
          </p>
<div class="note"><table border="0" summary="Note">
<tr>
<td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../../images/note.png"></td>
<th align="left">Note</th>
</tr>
<tr><td align="left" valign="top"><p>
              Starting with <a href="http://boost-spirit.com" target="_top">Spirit</a> V2.5
              (distributed with Boost V1.47) the placeholder <code class="computeroutput"><span class="identifier">_val</span></code>
              can be used in semantic actions attached to top level generator components
              as well. See <a class="link" href="../generate_api.html" title="Generator API">The
              Generator API</a> for more information.
            </p></td></tr>
</table></div>
<h6>
<a name="spirit.karma.reference.generator_concepts.nonterminal.h5"></a>
            <span class="phrase"><a name="spirit.karma.reference.generator_concepts.nonterminal._code__phrase_role__identifier___r1__phrase___code_____code__phrase_role__identifier__r10__phrase___code_"></a></span><a class="link" href="nonterminal.html#spirit.karma.reference.generator_concepts.nonterminal._code__phrase_role__identifier___r1__phrase___code_____code__phrase_role__identifier__r10__phrase___code_"><code class="computeroutput"><span class="identifier">_r1</span></code>...<code class="computeroutput"><span class="identifier">r10</span></code></a>
          </h6>
<p>
            The <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">spirit</span><span class="special">::</span><span class="identifier">_r1</span></code>...<code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">spirit</span><span class="special">::</span><span class="identifier">r10</span></code>
            placeholders can be used in <a href="../../../../../../../../libs/phoenix/doc/html/index.html" target="_top">Boost.Phoenix</a>
            semantic actions anywhere in the Nonterminal's definition. These <a href="../../../../../../../../libs/phoenix/doc/html/index.html" target="_top">Boost.Phoenix</a> placeholders
            refer to the Nonterminal's inherited attributes.
          </p>
<h6>
<a name="spirit.karma.reference.generator_concepts.nonterminal.h6"></a>
            <span class="phrase"><a name="spirit.karma.reference.generator_concepts.nonterminal.locals"></a></span><a class="link" href="nonterminal.html#spirit.karma.reference.generator_concepts.nonterminal.locals">Locals</a>
          </h6>
<p>
            Nonterminals can have local variables that will be created on the stack
            at runtime. A locals descriptor added to the Nonterminal declaration
            will give the Nonterminal local variables:
          </p>
<pre class="programlisting"><span class="keyword">template</span> <span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">T0</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">T1</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">T2</span><span class="special">,</span> <span class="special">...,</span> <span class="keyword">typename</span> <span class="identifier">TN</span><span class="special">&gt;</span>
<span class="keyword">struct</span> <span class="identifier">locals</span><span class="special">;</span>
</pre>
<p>
            where <code class="computeroutput"><span class="identifier">T0</span></code> ... <code class="computeroutput"><span class="identifier">TN</span></code> are the types of local variables
            accessible in your <a href="../../../../../../../../libs/phoenix/doc/html/index.html" target="_top">Boost.Phoenix</a>
            semantic actions using the placeholders:
          </p>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
<li class="listitem">
                <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">spirit</span><span class="special">::</span><span class="identifier">_a</span></code>
              </li>
<li class="listitem">
                <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">spirit</span><span class="special">::</span><span class="identifier">_b</span></code>
              </li>
<li class="listitem">
                <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">spirit</span><span class="special">::</span><span class="identifier">_c</span></code>
              </li>
<li class="listitem">
                <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">spirit</span><span class="special">::</span><span class="identifier">_d</span></code>
              </li>
<li class="listitem">
                <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">spirit</span><span class="special">::</span><span class="identifier">_e</span></code>
              </li>
<li class="listitem">
                <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">spirit</span><span class="special">::</span><span class="identifier">_f</span></code>
              </li>
<li class="listitem">
                <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">spirit</span><span class="special">::</span><span class="identifier">_g</span></code>
              </li>
<li class="listitem">
                <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">spirit</span><span class="special">::</span><span class="identifier">_h</span></code>
              </li>
<li class="listitem">
                <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">spirit</span><span class="special">::</span><span class="identifier">_i</span></code>
              </li>
<li class="listitem">
                <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">spirit</span><span class="special">::</span><span class="identifier">_j</span></code>
              </li>
</ul></div>
<p>
            which correspond to the Nonterminal's local variables <code class="computeroutput"><span class="identifier">T0</span></code>
            ... <code class="computeroutput"><span class="identifier">T9</span></code>.
          </p>
<div class="variablelist">
<p class="title"><b>Notation</b></p>
<dl class="variablelist">
<dt><span class="term"><code class="computeroutput"><span class="identifier">x</span></code></span></dt>
<dd><p>
                  A Nonterminal
                </p></dd>
<dt><span class="term"><code class="computeroutput"><span class="identifier">X</span></code></span></dt>
<dd><p>
                  A Nonterminal type
                </p></dd>
<dt><span class="term"><code class="computeroutput"><span class="identifier">arg1</span></code>, <code class="computeroutput"><span class="identifier">arg2</span></code>, ..., <code class="computeroutput"><span class="identifier">argN</span></code></span></dt>
<dd><p>
                  <a class="link" href="../basics.html#spirit.karma.reference.basics.lazy_argument">Lazy
                  Arguments</a> that evaluate to each of the Nonterminal's inherited
                  attributes.
                </p></dd>
</dl>
</div>
<h6>
<a name="spirit.karma.reference.generator_concepts.nonterminal.h7"></a>
            <span class="phrase"><a name="spirit.karma.reference.generator_concepts.nonterminal.valid_expressions"></a></span><a class="link" href="nonterminal.html#spirit.karma.reference.generator_concepts.nonterminal.valid_expressions">Valid
            Expressions</a>
          </h6>
<p>
            In addition to the requirements defined in <a class="link" href="generator.html" title="Generator"><code class="computeroutput"><span class="identifier">Generator</span></code></a>, for any Nonterminal
            the following must be met:
          </p>
<div class="informaltable"><table class="table">
<colgroup>
<col>
<col>
<col>
</colgroup>
<thead><tr>
<th>
                    <p>
                      Expression
                    </p>
                  </th>
<th>
                    <p>
                      Semantics
                    </p>
                  </th>
<th>
                    <p>
                      Return type
                    </p>
                  </th>
</tr></thead>
<tbody>
<tr>
<td>
                    <p>
                      <code class="computeroutput"><span class="identifier">x</span></code>
                    </p>
                  </td>
<td>
                    <p>
                      In a generator expression, invoke Nonterminal <code class="computeroutput"><span class="identifier">x</span></code>
                    </p>
                  </td>
<td>
                    <p>
                      <code class="computeroutput"><span class="identifier">X</span></code>
                    </p>
                  </td>
</tr>
<tr>
<td>
                    <p>
                      <code class="computeroutput"><span class="identifier">x</span><span class="special">(</span><span class="identifier">arg1</span><span class="special">,</span>
                      <span class="identifier">arg2</span><span class="special">,</span>
                      <span class="special">...,</span> <span class="identifier">argN</span><span class="special">)</span></code>
                    </p>
                  </td>
<td>
                    <p>
                      In a generator expression, invoke Nonterminal <code class="computeroutput"><span class="identifier">x</span></code> passing in inherited attributes
                      <code class="computeroutput"><span class="identifier">arg1</span></code>...<code class="computeroutput"><span class="identifier">argN</span></code>
                    </p>
                  </td>
<td>
                    <p>
                      <code class="computeroutput"><span class="identifier">X</span></code>
                    </p>
                  </td>
</tr>
<tr>
<td>
                    <p>
                      <code class="computeroutput"><span class="identifier">x</span><span class="special">.</span><span class="identifier">name</span><span class="special">(</span><span class="identifier">name</span><span class="special">)</span></code>
                    </p>
                  </td>
<td>
                    <p>
                      Set the name of a Nonterminal
                    </p>
                  </td>
<td>
                    <p>
                      <code class="computeroutput"><span class="keyword">void</span></code>
                    </p>
                  </td>
</tr>
<tr>
<td>
                    <p>
                      <code class="computeroutput"><span class="identifier">x</span><span class="special">.</span><span class="identifier">name</span><span class="special">()</span></code>
                    </p>
                  </td>
<td>
                    <p>
                      Get the name of a Nonterminal
                    </p>
                  </td>
<td>
                    <p>
                      <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span></code>
                    </p>
                  </td>
</tr>
</tbody>
</table></div>
<h6>
<a name="spirit.karma.reference.generator_concepts.nonterminal.h8"></a>
            <span class="phrase"><a name="spirit.karma.reference.generator_concepts.nonterminal.type_expressions"></a></span><a class="link" href="nonterminal.html#spirit.karma.reference.generator_concepts.nonterminal.type_expressions">Type
            Expressions</a>
          </h6>
<div class="informaltable"><table class="table">
<colgroup>
<col>
<col>
</colgroup>
<thead><tr>
<th>
                    <p>
                      Expression
                    </p>
                  </th>
<th>
                    <p>
                      Description
                    </p>
                  </th>
</tr></thead>
<tbody>
<tr>
<td>
                    <p>
                      <code class="computeroutput"><span class="identifier">X</span><span class="special">::</span><span class="identifier">sig_type</span></code>
                    </p>
                  </td>
<td>
                    <p>
                      The Signature of <code class="computeroutput"><span class="identifier">X</span></code>:
                      In a function signature form as described above in the Signature
                      paragraph.
                    </p>
                  </td>
</tr>
<tr>
<td>
                    <p>
                      <code class="computeroutput"><span class="identifier">X</span><span class="special">::</span><span class="identifier">locals_type</span></code>
                    </p>
                  </td>
<td>
                    <p>
                      The local variables of <code class="computeroutput"><span class="identifier">X</span></code>:
                      An <a href="../../../../../../../../libs/mpl/doc/refmanual/forward-sequence.html" target="_top">MPL
                      Forward Sequence</a>.
                    </p>
                  </td>
</tr>
</tbody>
</table></div>
<h6>
<a name="spirit.karma.reference.generator_concepts.nonterminal.h9"></a>
            <span class="phrase"><a name="spirit.karma.reference.generator_concepts.nonterminal.models"></a></span><a class="link" href="nonterminal.html#spirit.karma.reference.generator_concepts.nonterminal.models">Models</a>
          </h6>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
<li class="listitem">
                <a class="link" href="../nonterminal/rule.html" title="Generator Rule">rule</a>
              </li>
<li class="listitem">
                <a class="link" href="../nonterminal/grammar.html" title="Generator Grammar">grammar</a>
              </li>
</ul></div>
</div>
<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
<td align="left"></td>
<td align="right"><div class="copyright-footer">Copyright © 2001-2011 Joel de Guzman, Hartmut Kaiser<p>
        Distributed under the Boost Software License, Version 1.0. (See accompanying
        file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
      </p>
</div></td>
</tr></table>
<hr>
<div class="spirit-nav">
<a accesskey="p" href="narygenerator.html"><img src="../../../../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../generator_concepts.html"><img src="../../../../../../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../../../../index.html"><img src="../../../../../../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="../basics.html"><img src="../../../../../../../../doc/src/images/next.png" alt="Next"></a>
</div>
</body>
</html>
